Hydraulic circuits typically include a hydraulic reservoir of fixed volume, a pump for circulating the hydraulic fluid within a specific circuit, a filter and a cooler. The volume of the hydraulic reservoir is typically defined in accordance with the pumping rate of the pump. In general, the capacity of the reservoir is two to three times greater than the pumping rate of the pump and sometimes even more. This results in bulky reservoirs. Furthermore, the presence of air in hydraulic fluid is often problematic. For instance, the air may contaminate and oxidize the hydraulic fluid, cause pump cavitation problems, and may represent a risk of fire hazard.
Accordingly, efforts have been made to isolate the reserve of fluid of a hydraulic system from the atmosphere and the surrounding medium. For instance, U.S. Pat. No. 3,099,189, issued on Jul. 30, 1963 to Blondiau, discloses a fluid reservoir having a hollow body for containing a fluid and an elastic diaphragm adapted to fit within the hollow body to exert a pressure on the fluid. The bottom surface of the diaphragm follows the fluid level, according to the demand from the hydraulic circuits connected to the reservoir.
The AMSAA technical report No. 426 entitled “Hydraulic Design Guidebook Survivability And System Effectiveness” that was published by the Fluid Power Research Center Of the Oklahoma State University in August 1986 discloses a critical volume reservoir (CVR) comprising a cylindrical vessel and a piston that is axially slidable in the cylindrical vessel. The piston divides the interior space of the cylindrical vessel into first and second variable volume chambers. The first chamber is connected in fluid flow communication with a hydraulic system. The second chamber houses a compression spring acting on the piston to resist movement thereof under the pressure exerted thereon by the fluid in the first chamber. The force of reaction induced in the spring is directly transmitted from the piston to the top cover plate of the cylindrical vessel. The top cover plate must therefore be of sturdy construction. The fact that the spring is located within the cylindrical vessel also contributes to increasing the space occupied by the reservoir.
Although the variable volume reservoirs disclosed in the above-mentioned documents permits isolating the hydraulic fluid from the atmosphere, it has been found that there is still a need for a new lightweight and compact reservoir that is adapted to feed a hydraulic fluid under pressure to a hydraulic system, without inducing additional mechanical stress in the structure of the reservoir.